Have been trying IF for a while now under Hofmekler's Warrior Diet program. Been on and off IF, but do notice that on IF days I feel light and energetic. However, sometimes I become too excited and anxious. Also, as I tend to do a lot of mental work, I encounter mental blocks about once every 2 hours, which are remedied by a piece of fruit or some fresh juice.

You drinking coffee or other stimulants? That can cause anxiety especially while fasting when your SNS nervous system is already running....like throwing more gasoline on a fire.

Also it's not good as it will raise cortisol....which it not a goal as well.

Dizziness and brain fog may be a sign of low blood sugar....as your brain is a glucose hog. Hence why you have sugar (fruits, drinks) and you are fine. This is something that can get better over time as well once you start turning the tide on insulin resistance.